In my opinion, I think hyorin of Sistar has the best voice in the K-Pop Industry, at the moment. Some people think IU. But the range and dynamics of Hyorin's voice, I think, outweigh that of IU's by far.
Below is IU. I tried to find the best of IU, so as to not skew your opinion:
http://www.vingle.net/posts/9431-IU-Powerful-singer-or-not